The concept of color temperature is based on the comparison of a visible light source to that of an ideal black-body radiator. The color temperature (CT) scale assigns numerical values to the color emitted by the black-body source being heated, measured in degrees Kelvin (K). A black-body radiator is a theoretical material having perfect absorptivity at all wavelengths, making it the best possible emitter of thermal radiation.
One typically thinks of color temperature in terms of the “whiteness” versus “warmness” of the light source, i.e., “Daylight White” is 5000-6500 K, “Cool White” is 3500-5000 K, “Warm White” is 3500 K and below. Thermal light sources, such as incandescent lamp filaments, are called “incandescent radiators”, and have a spectral power distribution (SPD) more uniformly spread throughout the visible spectrum. Light sources that are not incandescent radiators, which do not exhibit visible spectrum radiation at all wavelengths of the spectrum, are measured according to a “correlated color temperature” (CCT) scale. The CCT scale is adjusted according to human perception. White LEDs are measured using the CCT scale.
It is well-known that the color of the light produced by incandescent lamps changes when the lamp is dimmed. When the incandescent lamp is at full rated power, its correlated color temperature (CCT) is usually within the range of 2700 K-3300 K. However, when the incandescent lamp is dimmed, the CCT changes to as low as 1700 K, approximately the color of a match flame. To the human eye, the incandescent bulb appears to go from white to yellow, or to give a “warm glow” when dim. For many years, this inherent characteristic of incandescent bulbs has been seen as a feature used with AC dimmer modules to create a warm and cozy environment in restaurants, hotels, theaters, homes, etc.
LED light fixtures, which became popular in recent years because of their energy efficiency, give off light that does not normally change color when the fixture is dimmed. The white light from an LED light source has a constant CCT across the entire range of the AC dimmer module. Hence, even at lower light levels when dimmed, the white light given off by LED light fixtures has been perceived as unusual or unnatural, particularly when used in the aforementioned environments when trying to create a warming lighting effect. LED lighting manufacturers have been aware of this problem, but have yet to find a way to accurately duplicate the “warm glow” dimming characteristics of incandescent bulbs in a cost-effective manner.
One way to simulate the “warming with dimming” characteristic of an incandescent lamp with an LED light source is to optically mix white LEDs with amber (yellow/orange) LEDs, and control their currents in such a manner that the mixed color light from the LED combination changes from the 3000K white light to a more amber-yellow-white color with dimming. Traditionally, LED systems performing mixing of two or more colored LEDs use individual drivers controlling each colored LED separately, or use a single driver designed to have two or more separate output channels, where each output channel is controlled individually within the driver. U.S. Pat. No. 7,288,902 to Melanson, incorporated by reference herein, describes such a circuit having multiple light sources to vary the color temperatures in response to changing dimming levels. The light source driver provides individual drive currents to each light source in response to the selected dimming level in order to achieve the desired color temperature. At least a two-channel LED driver must be used to power an amber LED in addition to a white LED array in this circuit.
The use of individual LED drivers, or a multi-channel output LED driver, to control each LED colored array, has several disadvantages. First, and most importantly, is the additional cost. As LED lighting becomes more prevalent, designers seek the most cost-effective alternatives. Second, complexity and reliability issues demand the simplest solutions. Custom multiple-channel LED drivers designed for specific types of dimmers are not a preferred lighting solution. Third, if the warming-while-dimming function could be accomplished inside the LED light engine using only the two inputs provided by a single-channel LED driver, the LED light engine could be used with most any LED driver, and dimmable with most any AC dimmer module.
A need, therefore, exists for an LED circuit arrangement and LED light fixture that can be used with a single-channel variable-DC LED driver to simulate the color temperature changes of incandescent lamps as dimming levels change.